1. Field of Invention
The present invention relates to non-heat treated steel which can exhibit improved toughness in an ascooled state without being subjected to quenching and tempering treatments and which can be used as a material for producing mechanical parts.
2. Description of the Related Art
Mechanical parts such as automobile parts are usually produced from steel bar by hot-forging, quenching and tempering treatments followed by machining. In the production of such parts, from the viewpoint of energy conservation and a reduction in the production cost of the parts, direct quenching after forging using the heat remaining after hot-forging or non-heat treated steel, wherein precipitation hardening of V, Nb or the like is utilized, is widely known as a technique omitting heat treatment (e.g., Automobile Technique, Vol. 37, No. 3, p 242, 1983, or Japanese Unexamined Patent Publication (Kokai) No 55-82749). However, the non-heat treated steel to which V, Nb or the like is added, i.e., in which a so-called micro alloying technique is utilized exhibits low toughness, because of coarse grained ferrite-pearlite structure in an as-spontaneously-cooled state after hot forging, due to the character of the form in practical use, and therefore, that type of steel is limited in the scope of practical application. Thus, at present, the non-heat treated steel is not used as a material for important safety-preservation parts an automobile, e.g., the suspension and associated members thereof.
The toughness of this conventional non-heat treated steel is variable depending on the size of the part, the desired level of strength, the forging method and conditions and the like. Usually, the impact value determined at 25.degree. C. using an impact specimen according to JIS No. 3 is as low as 5 kg-m/cm.sup.2 or less. The toughness can be increased by reducing the heating and finishing temperatures during the forging, so as to refine the crystal grains. However, this temperature reduction involves problems such as the life of the forging dies and the degree of filling in of the dies.